This invention relates to phosphors. More particularly, this invention is concerned with a method for applying a coating to the outer surface of individual phosphor particles.
Phosphors are used in mercury vapor discharge lamps and viewing screens of various electronic devices. It has been recognized that various improvements in the performance of phosphors can be obtained if the phosphor material is coated with a protective film or pigment. Numerous attempts have been made to coat the outer surface of individual phosphor particles with a protective coating. Such coatings having been applied, for example, by using selective additions to a lamp coating suspension; or by suspending particles of the phosphor in a solution containing the coating material and evaporating the solvent to form coated phosphor particles.
Disadvantages associated with these known methods are eliminated by the method of the present invention by which a continuous coating having a desired substantially uniform thickness is formed on the outer surface of the phosphor particles. In a preferred embodiment, the coating is formed by a gas-phase chemical vapor deposition of the coating material on the phosphor particles while the particles are suspended in a fluidized bed which is maintained in a temperature gradient.